Validation on Execution of workitem

Hi all,

I have a scenario in which a workitem with expense value 7000$ has gone for approval to manager1 in his UWL. Manager1 can approve expense upto 10000$ But when this workitem was created Manager1 has gone on leave and he has substituted Manager2 for this approval. Manager2 has limit of approving expenses only upto 5000$.

So what I want is when this Manager 2 click on approval of this workitem in his UWL the validation should happen and it should give Manager2 an Error message that ' 7000$ is higher than your approval limit' and workitem should not get executed and should remain in Manager2 UWL so that later when Manager1 comes Manager2 can forward this workitem back to Manager1

How to do this...should i do this validation in my BO method then how to throw error in UWL screen. else if this validation is to be done in JAVA or ABAP Webdynpro of UWL or any other way to achieve this...

Thanks in advance..